Although I'm not a gar expert, that looks like a small alligator gar to me.

At first i thought u were going to show a picture of a melanistic fish, but it looks more or less that its just a darker then normal gar. Could be from living in a darker habitat, something odd about its diet, or even though i don't think so it could actually be melanistic. More then likely its a combination of the first two.

Actually the Black Phase of a gar is sort of like an albino/golden rainbow or a all white channel cat, its not a true albino but a, 1 in a thousand color scheme that happens even in the wild, more are caught because the black phase is less obvious to predators then say a true albino. If you look at pet shops, they sometimes get "black " gar and they fetch a high price for the mere oddity of them. More then likely it was a Long Nose but from the Gar research I have done over the years it is also prevelent in Aligators and Shornose. Way cool catch and if you get another one, small enough,kept alive, I bet he would go for a pretty penny to a well heeled aquarium type.